1. Industrial Field of the Invention
The present invention relates to a suspension device provided between the wheel (or the axle) and the body of a vehicle, and in particular, to a suspension control device suitable as a semi-active suspension which makes the damping coefficient change continuously according to the vibration state of the vehicle.
2. Related Art
Conventionally, some proposals with respect to improvements of the vibration transmission characteristics of a suspension which can make the damping coefficient change according to the state of the Vertical vibration of the vehicle, are disclosed in the U.S. Pat. No. 3,807,678, on pages 619-626 of "ASME, Journal of Engineering for Industry" No. 96-2, published in May, 1974, and the like. As described in the documents, a method is known for controlling the coefficient by judging the sign of the product of the absolute velocity S of a sprung mass (a body) which is the velocity of the vertical vibration of the body and the relative velocity of the sprung mass (the body) to the unsprung mass (a wheel). A method for controlling the coefficient by judging the sign of the product of the relative displacement of the sprung mass (the body) to the unsprung mass (the wheel) and the relative velocity thereof, is known, as described in the U.S. Pat. No. 4,821,849.
The former control method will be briefly explained as follows.
In the theory of damping, it has been known that good damping characteristics are obtained by providing a shock absorber which generates a damping force to the absolute velocity S of the sprung mass (the body), between the sprung mass (the body) and a point restricted by the absolute coordinate system. However, in a car, it is impossible to attach a shock absorber to the absolute coordinate system in practice. Therefore, it is considered to approximate that by providing a shock absorber between the sprung mass (the body) and the unsprung mass (the wheel) in parallel so that the damping force of the shock absorber is variable. In this case, the shock absorber provided between the sprung mass and the unsprung mass (the wheel) generates damping force in only the direction contrary to the extension or the compression of the shock absorber. Accordingly, the shock absorber sometimes cannot generate the damping force in the same direction as that of the shock absorber provided between the sprung mass and the absolute coordinate system. Therefore, the damping force at that time is deemed to be zero.
The above concept is shown in equation form as follows. EQU IF S(S-X)&gt;0 (1) EQU F=-CsS=-C(S-X) (2) EQU C=CsS/(S-X) (3) EQU IF S(S-X)&lt;0 (4) EQU F=0 (5) EQU C=0 (6)
wherein,
S: absolute velocity of the sprung mass (the body); PA1 X: absolute velocity of the unsprung mass (the wheel); PA1 F: damping force of the shock absorber; PA1 Cs: damping coefficient of the shock absorber provided between the sprung mass and the absolute coordinate system; PA1 C: damping coefficient of the shock absorber provided between the sprung mass and the unsprung mass (the wheel). PA1 a controller for determining the absolute velocity of the vertical vibration of the body on the basis of the detected signal from the vertical vibration detecting means, wherein the controller outputs a control signal to the shock absorber of the variable damping coefficient type so that the control signal makes the damping coefficient during compression have a small value and makes the damping coefficient during extension have a large value when it is judged that the body is moving in the upper direction on the basis of the absolute velocity, and the control signal makes the damping coefficient during extension have a small value and makes the damping coefficient during compression have a large value when it is judged that the body is moving in the lower direction on the basis of the absolute velocity.
Therefore, it is possible to obtain good damping characteristics similar to that of the shock absorber provided between the sprung mass and the absolute coordinate system, by controlling the damping coefficient C of the shock absorber provided between the sprung mass and the unsprung mass (the wheel) according to the equations (3) and (6), and to the conditions as shown in the equations (1) and (4).
However, the above described techniques require measuring the relative displacement between the body of the sprung mass and the wheel of the unsprung mass, or the relative velocity between them along the vertical direction. Therefore, in order to use such a technique for a vehicle, a vehicle height sensor for detecting the distance between the body and the wheel generally had to be attached under the body.
When a vehicle having such a vehicle height sensor is used in an area where it snows, snow often adheres to the height sensor during travelling, and freezes it. When one begins to operate the vehicle the next morning, the destruction of the height sensor often occurs due to operating the linkage or the like of the height sensor by force.